1. create a mask and click OK
2. click on the mask thumbnail next to the output -> the preview gets split
3. close the output with the mask -> the preview stays split with the left half empty
The preview window stays split even if you close the input too and open a new image.

The preview also stays split if you close the input instead of output in the step 3.

If you click to the empty half, you get the error message: empty input image.

To un-split the preview window, open any transformation and run it or just cancel it.